JP2017031947A - Low-pressure steam turbine structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-pressure steam turbine structure capable of achieving excellent diffuser effect without having a complicated mechanism.SOLUTION: In a low-pressure steam turbine structure including a turbine rotor, a plurality of final-stage moving blades disposed in a circumferential direction of the turbine rotor, a cover disposed on outer peripheral tip portions of the final-stage moving blades, and an exhaust diffuser disposed at a downstream side of the final-stage moving blades and configuring a steam main flow channel, a meridian plane of an inner peripheral-side wall surface of the exhaust diffuser corresponding to an outer peripheral face of the main steam flow channel, is formed into the S-shape projecting to an inner peripheral side from a downstream side end of the cover toward a downstream direction, and a radial position of an innermost peripheral side of the outer peripheral face of the steam main flow channel at the downstream side with respect to the downstream side end of the cover is determined at an outer peripheral side with respect to a line segment obtained by extending an inner peripheral face of the cover in a tangential direction.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a low pressure steam turbine structure.

  A power plant that generates power by rotating a turbine with steam generated by a steam generator such as a boiler is equipped with steam turbines such as a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine in this order. Then, it is introduced into the condenser via the exhaust chamber, condensed in the condenser, becomes condensed water, and returns to the steam generator. Immediately after the high-pressure, medium-pressure, and low-pressure turbine outlets, there is a steam passage called an exhaust chamber, which generally has a shape that involves a sudden flow diversion, resulting in resistance to steam flow and pressure loss. It is easy to produce.

  In particular, the pressure loss in the low-pressure exhaust chamber, which is the steam flow path from the outlet of the low-pressure turbine to the condenser, has a great influence on the plant performance, and reducing this pressure loss is effective for improving the plant performance. In addition, the exhaust chamber provided at the outlet of the low-pressure turbine has a diffuser channel that expands the steam channel toward the condenser, and the pressure drop due to the diffuser effect increases the heat drop of the turbine. The output can be improved.

  When the diffuser effect is effectively exhibited, the outlet pressure of the low-pressure turbine decreases, and the steam pressure difference at the low-pressure turbine inlet / outlet increases, so that a higher output can be obtained. Among the power plants having such a configuration, there is a lower exhaust type plant in which a condenser is placed below a low-pressure turbine to reduce the size of a building that houses the power plant. In the lower exhaust type exhaust chamber, the steam exhausted from the low-pressure turbine bends downward toward the condenser at a short distance, and therefore mixes smoothly without being bent, and the loss tends to increase. For this reason, an annular member, usually called a steam guide (flow guide), is attached to the outlet of the final stage of the low-pressure turbine to suppress the deterioration of performance by assisting the smooth flow of steam. Furthermore, by forming a diffuser flow path with a flow path sandwiched between the steam guide and the wall surface on the rotor side, performance improvement by pressure recovery is achieved.

  By the way, when the gas flows through the pipeline, a boundary layer is developed in the vicinity of the wall surface due to friction between the wall surface and the gas or the like, which is a region where the average flow velocity is low. When an exhaust chamber member such as a steam guide is formed with a large curvature, it becomes difficult for gas to flow along the wall surface due to the influence of centrifugal force, and a boundary layer grows greatly in the vicinity of the wall surface. When the flow velocity near the wall surface is further slowed down and the boundary layer grows greatly, a vortex or reverse flow is finally generated near the wall surface. Such a flow state is referred to as flow separation.

  When such flow separation occurs, the gas does not flow along the wall surface, and the effective flow path area is reduced, so that the diffuser effect in the exhaust chamber is reduced. Therefore, there is a problem that sufficient pressure recovery cannot be ensured and the efficiency of the low-pressure turbine is reduced. The boundary layer is a region having a flow velocity of 99% or less of the main flow average flow velocity. Further, the fact that the boundary layer is greatly developed means that the thickness of the boundary layer from the wall surface is increased.

  In order to solve such a problem, for example, Non-Patent Document 1 discloses a technique for increasing the flow velocity of the gas flowing near the wall surface by sucking the gas flowing near the wall surface into the wall surface in the direction along the flow direction, and A technique is described in which a blowing duct is provided on a wall surface, and a gas is blown out in a direction along the flow direction to increase the flow velocity of the gas flowing in the vicinity of the wall surface.

Shirakura, Ohashi, “Hydrodynamics (2)”, Corona, 1969, P123-125

  However, for example, when the technique described in Non-Patent Document 1 is applied to the exhaust chamber of a low-pressure steam turbine, a mechanism for sucking in gas (steam) or a mechanism for blowing out gas is required. There arises a problem that the structure of this becomes complicated.

  The present invention has been made based on the above-described matters, and an object thereof is to provide a low-pressure steam turbine structure capable of obtaining a large diffuser effect without having a complicated mechanism.

  In order to solve the above problems, for example, the configuration described in the claims is adopted. The present application includes a plurality of means for solving the above-described problems. To give an example, a turbine rotor, a plurality of final stage moving blades provided in the circumferential direction of the turbine rotor, and the last stage moving blades are provided. A low-pressure steam turbine structure including a cover provided at an outer peripheral tip and an exhaust diffuser provided downstream of the final stage moving blade and forming a steam main flow path, wherein the outer peripheral surface of the main steam flow path The meridian shape of the inner peripheral side wall surface of the exhaust diffuser corresponding to is formed in an S-shape projecting toward the inner peripheral side from the downstream end of the cover toward the downstream side, and downstream of the cover A radial position on the innermost peripheral side of the outer peripheral surface of the steam main flow channel downstream from the side end is provided on the outer peripheral side from a line segment obtained by extending in a tangential direction of the inner peripheral surface of the cover. And

  According to the present invention, it is possible to provide a low-pressure steam turbine structure capable of obtaining a large diffuser effect without having a complicated mechanism.

It is sectional drawing which shows the meridian cross section of the last stage of the steam turbine which is 1st Embodiment of the low pressure steam turbine structure of this invention, and an exhaust diffuser. It is sectional drawing which shows the meridian section of the last stage of the conventional steam turbine, and an exhaust diffuser. It is a conceptual diagram which shows the boundary layer velocity distribution with respect to the flow position of the exhaust diffuser of the conventional steam turbine. It is a conceptual diagram which shows the boundary layer speed distribution with respect to the flow position of the exhaust diffuser of the steam turbine which is 1st Embodiment of the low pressure steam turbine structure of this invention. It is sectional drawing which shows the meridional section of the final stage of the steam turbine which is 2nd Embodiment of the low pressure steam turbine structure of this invention, and an exhaust diffuser. It is a conceptual diagram which shows a part of meridional section of the final stage of the steam turbine which is 3rd Embodiment of the low pressure steam turbine structure of this invention, and an exhaust diffuser. It is a conceptual diagram which shows the boundary layer speed distribution with respect to the flow position of the exhaust diffuser of the steam turbine which is 4th Embodiment of the low pressure steam turbine structure of this invention.

  Embodiments of the low-pressure steam turbine structure of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional view showing a meridional section of a final stage of a steam turbine and an exhaust diffuser according to a first embodiment of the low-pressure steam turbine structure of the present invention.
As shown in FIG. 1, a stationary blade 1 and a moving blade 2 form a pair to constitute a turbine final stage. The outer peripheral end of the stationary blade 1 is supported by an outer peripheral side diaphragm 3 fixed to a casing (not shown), and the inner peripheral end is provided with an inner peripheral side diaphragm 4. A plurality of stationary blades 1 are provided in the turbine circumferential direction. A plurality of rotor blades 2 are fixed to the turbine rotor 5 in the circumferential direction, and the cover 6 at the outer peripheral end of the rotor blade 2 is connected to each other by twisting back due to centrifugal force during operation. The turbine rotor 5 includes a central shaft 11.

  Arrow 7 indicates the flow direction of the steam flowing into the final stage of the steam turbine, and arrow 8 indicates the flow direction of the steam flowing out from the final stage of the steam turbine. Hereinafter, the downstream side in the steam flow direction is simply referred to as the downstream side, and the upstream side in the steam flow direction is simply referred to as the upstream side.

  A flow guide portion 9 is formed at the downstream end of the outer peripheral diaphragm 3 to guide the steam that has exited the rotor blade 2 to an exhaust chamber (not shown). A bearing cone 12 is formed on the inner circumferential side of the flow guide portion 9 in the turbine radial direction (hereinafter simply referred to as the inner circumferential side), and steam called an exhaust diffuser 16 is formed between the bearing cone 12 and the flow guide portion 9. A main flow path is formed.

  The flow guide portion 9 and the bearing cone 12 are each curved in the turbine radial direction, and the exhaust diffuser 16 expands in diameter toward the downstream side and communicates with the exhaust chamber. For this reason, the steam that has passed through the moving blade 2 in the final paragraph passes through the exhaust diffuser 16 and decelerates while changing the flow direction from the axial direction to the radial direction, and the energy of the deceleration is converted into pressure. And then led to the exhaust chamber.

  In the present embodiment, the meridian shape of the inner peripheral side wall surface of the flow guide portion 9 constituting the exhaust diffuser 16 is increased in diameter from the downstream side portion of the cover 6 at the outer peripheral side tip of the moving blade 2 from the downstream side end. The main feature is that it is formed in an S shape that is convex toward the inner peripheral side toward the portion, and this convex portion is a reduced diameter portion G. Here, the reduced diameter portion G is formed on the outer peripheral surface of the steam main flow path that is the exhaust diffuser 16 (hereinafter referred to as the steam main flow path outer peripheral surface 10).

  Specifically, in the meridional section of the inner peripheral side wall surface of the flow guide portion 9 shown in FIG. Here, Ra indicates a diameter at a point located on the most upstream side of the meridional section of the inner peripheral side wall surface of the flow guide portion 9 that is the downstream end of the cover 6 at the outer peripheral end of the rotor blade 2, and Rb indicates the contraction. The diameter of the most reduced diameter part which becomes the minimum diameter in the diameter part G is shown.

  Next, in order to facilitate understanding of the operational effects of the present invention in the present embodiment, the flow state of the exhaust diffuser in the conventional steam turbine will be described with reference to FIGS. 2 and 3. FIG. 2 is a sectional view showing a meridional section of a final stage of a conventional steam turbine and an exhaust diffuser, and FIG. 3 is a conceptual diagram showing a boundary layer velocity distribution with respect to a flow position of the exhaust diffuser of the conventional steam turbine.

  FIG. 2 shows the final stage of the conventional steam turbine and the exhaust diffuser. The only difference from the embodiment of the present invention is the presence or absence of the reduced diameter portion G. In the case of the conventional example, as shown in FIG. 2, the reduced diameter portion G is not provided on the outer peripheral surface 10 of the steam main flow path on the downstream side of the downstream end of the cover 6 at the outer peripheral end of the moving blade 2.

  FIG. 3 shows the boundary layer velocity distribution with respect to the flow position of the exhaust diffuser in such a conventional steam turbine. In FIG. 3, 15a shows the boundary layer velocity distribution in the position of the vicinity of the downstream end of the cover 6 of the moving blade 2, and shows the boundary layer velocity distribution in the downstream position in order of 15b, 15c, 15d. Most of the steam that flows into the final stage of the turbine and flows out of the stationary blade 1 flows out after flowing into the moving blade 4 as the main steam flow 14. And a part flows out after flowing in into the leakage flow path formed between the cover 6 of the moving blade 4 and the outer peripheral diaphragm 3 as the steam leakage flow 13.

  As shown in FIG. 3, in the boundary layer velocity distribution 15 a immediately after the downstream of the moving blade 2, a wake behind the cover 6 is generated by the cover 6 disposed at the outer peripheral tip of the moving blade 2, and a dead water region 17 is formed. Therefore, the speed deficit part appears. The velocity deficient portion appearing in the boundary layer velocity distribution of 15a becomes smaller because energy is supplied from the velocity field vertically above and below the flow direction as it goes downstream.

  However, the more the jet from the steam leak flow 13 is separated from the downstream end of the cover 6 of the rotor blade 2, the more energy is taken away, so the velocity in the vicinity of the steam main flow outer peripheral surface 10 decreases. Specifically, the speed decreases from 15a to 15c, and separation occurs due to backflow at 15d, which is the position of the exhaust diffuser outlet.

  Next, the details of the configuration and operation in this embodiment will be described with reference to FIGS. FIG. 4 is a conceptual diagram showing the boundary layer velocity distribution with respect to the flow position of the exhaust diffuser of the steam turbine which is the first embodiment of the low-pressure steam turbine structure of the present invention.

  As shown in FIGS. 1 and 4, in the present embodiment, the meridian shape of the inner peripheral side wall surface of the flow guide portion 9 constituting the exhaust diffuser 16 is set downstream of the cover 6 at the outer peripheral end of the rotor blade 2. The main feature is that it is formed in an S-shape that is convex toward the inner peripheral side from the downstream side of the side end toward the enlarged diameter portion, and this convex portion is the reduced diameter portion G. Further, the radial position on the innermost peripheral side of the outer peripheral surface 10 of the steam main flow channel downstream from the downstream end of the cover 6 of the final stage moving blade 2 is obtained by extending in the tangential direction of the inner peripheral surface of the cover 6. It is provided on the outer peripheral side from the line segment L1. By providing these features, it is possible to suppress the occurrence of peeling in the above-described conventional example.

  In FIG. 4, most of the steam flowing into the turbine final stage and flowing out from the stationary blade 1 flows into the moving blade 2 as the main steam flow 14 and then flows out into the exhaust diffuser 16. A part of the gas flows into a leak passage formed between the cover 6 of the rotor blade 2 and the outer diaphragm 3 as a steam leak flow 13 and then flows out to the exhaust diffuser 16.

  In the boundary layer velocity distribution 15a immediately after the moving blade 2 in the boundary layer velocity distribution with respect to the flow position of the exhaust diffuser shown in FIG. Is generated and a dead water region 17 is formed, so that the velocity deficit portion 17 appears.

  Further downstream, the steam main flow path outer peripheral surface 10 is formed in a convex shape on the inner peripheral side, so that the steam leak flow 13 moves toward the inner diameter side so as to fill the dead water region 17. Then, at the most reduced diameter portion (position of the boundary layer velocity distribution 15b) of the reduced diameter portion G of the outer peripheral surface 10 of the steam main flow path, the steam leak flow 13 and the steam main flow 14 merge, and the dead water region 17 is almost extinguished. Yes.

  In the enlarged diameter portion downstream from the position 15b, a boundary layer develops due to the frictional force between the steam main flow passage outer peripheral surface 10 and the working steam. However, the boundary layer internal flow in the vicinity of the outer peripheral surface 10 of the steam main channel has sufficient energy because the dead water region disappears due to the convex portion on the inner peripheral side, and therefore the exhaust diffuser outlet (position of the boundary layer velocity distribution 15d) ) Can flow out without peeling. As a result, the steam flow at the outlet of the exhaust diffuser can achieve higher efficiency than the prior art steam turbine that is relatively easy to peel off.

  Further, since the radial position of the most reduced diameter portion in the reduced diameter portion G is provided on the outer peripheral side from the line segment L1 obtained by extending in the tangential direction of the inner peripheral surface of the cover 6, the main steam flow 14 and the steam leakage flow 13, the dead water region 17 downstream of the cover 6 can be eliminated, and an increase in loss due to excessive interference between the steam main flow 14 and the steam leakage flow 13 can be suppressed.

  Further, the meridian shape of the inner peripheral side wall surface of the flow guide portion 9 constituting the exhaust diffuser 16 is changed from the downstream end of the cover 6 at the outer peripheral end of the rotor blade 2 toward the enlarged diameter portion. The steam leakage flow 13 can smoothly join the steam main flow 14 by forming the S-shape that is convex toward the inner peripheral side. As a result, mixing loss due to excessive interference between the two can be suppressed.

  According to the first embodiment of the low-pressure steam turbine structure of the present invention described above, it is possible to provide a low-pressure steam turbine structure capable of obtaining a large diffuser effect without having a complicated mechanism.

  Hereinafter, a second embodiment of the low pressure steam turbine structure of the present invention will be described with reference to the drawings. FIG. 5 is a sectional view showing the meridional section of the final stage of the steam turbine and the exhaust diffuser which is the second embodiment of the low-pressure steam turbine structure of the present invention. In FIG. 5, the same reference numerals as those shown in FIGS. 1 to 4 are the same parts, and detailed description thereof is omitted.

  Although the second embodiment of the low-pressure steam turbine structure of the present invention shown in FIG. 5 is configured with almost the same equipment as the first embodiment, the following configuration is different. The present embodiment is different in that the inner peripheral surface of the cover 6 at the tip on the outer peripheral side of the rotor blade 2 is inclined in the diameter increasing direction toward the downstream direction (having a slant angle). Here, hereinafter, the slant angle refers to an angle formed by the inner peripheral surface of the cover 6 and the turbine central shaft 11. In other words, the slant angle of the cover 6 in the first embodiment is 0 degree, whereas the present embodiment is different in that it has a slant angle.

  Also in the present embodiment, as shown in FIG. 5, from the line segment L2 obtained by extending the radial position of the most reduced diameter portion in the reduced diameter portion G in the tangential direction of the inner peripheral surface of the cover 6. It is provided on the outer peripheral side.

  In the present embodiment, since the inner peripheral surface of the cover 6 has a slant angle, the steam main flow 14 and the steam leakage flow 13 can smoothly merge in the vicinity of the downstream end of the cover 6. As a result, since the generation of the dead water region 17 is further improved, it is possible to provide a steam turbine that hardly causes separation at the outlet of the exhaust diffuser 16.

  According to the second embodiment of the low-pressure steam turbine structure of the present invention described above, the same effects as those of the first embodiment can be obtained.

  Moreover, according to 2nd Embodiment of the low pressure steam turbine structure of this invention mentioned above, since the slant angle was provided in the internal peripheral surface of the cover 6, generation | occurrence | production of the dead water area | region 17 can be suppressed more effectively. As a result, it is possible to provide a steam turbine in which separation at the outlet of the exhaust diffuser 16 hardly occurs.

  Hereinafter, a third embodiment of the low-pressure steam turbine structure of the present invention will be described with reference to the drawings. FIG. 6 is a conceptual view showing a part of the meridional section of the final stage of the steam turbine and the exhaust diffuser which is the third embodiment of the low-pressure steam turbine structure of the present invention. In FIG. 6, the same reference numerals as those shown in FIGS. 1 to 5 are the same parts, and detailed description thereof is omitted.

  Although the third embodiment of the low-pressure steam turbine structure of the present invention shown in FIG. 6 is configured with almost the same equipment as the first embodiment, the following configuration is different. The present embodiment is different in that the S-shape in the meridional shape of the inner peripheral side wall surface (steam main flow channel outer peripheral surface 10) of the flow guide portion 9 constituting the exhaust diffuser 16 is formed by a combination of three arcs. . Details of the configuration of the present embodiment will be described focusing on the differences from the first embodiment.

  The present embodiment is characterized in that the S-shaped wall surface, which is the meridional shape of the outer peripheral surface 10 of the steam main flow path, is composed of three arcs. In FIG. 6, the S-shape in the meridional shape of the inner peripheral side wall surface of the flow guide portion 9 that is the outer peripheral surface 10 of the steam main flow path is the first arc portion C1, the second arc portion C2, and the third arc portion C3. It consists of and.

  First, a point at a distance of a predetermined radius R1 vertically below the point A18 located at the most upstream of the meridional section on the inner peripheral side wall surface of the flow guide portion 9 is set as a center point O1, and a center angle α1 is centered on O1. When the point reached by the trajectory is B19, the portion formed by the arcs 18 and 19 becomes the first arc portion C1.

  Next, a point that is on a straight line extending from the center point O1 and the point B19 and that has a radius R1 from the point B19 is defined as a center point O2, and a point that reaches the center angle α2 from the point B19 with O2 as the center. Assuming D20, the portion formed by the arcs 19 and 20 is the second arc portion C2.

  Next, a point on a straight line extending from the point D20 and the center point O2, a point having a radius R3 from the point D20 is set as the center point O3, and a point reaching from the point D20 along the locus of the center angle α3 with O3 as the center. Assuming E21, the portion formed by the arcs 20 and 21 is the third arc portion C3.

  In the present embodiment, an S-shaped wall surface that is a meridional surface shape of the outer peripheral surface 10 of the steam main flow path can be formed by combining three arc portions in this way. There is an advantage that a smooth flow path can be easily formed and shape inspection is easy.

  According to the third embodiment of the low-pressure steam turbine structure of the present invention described above, the same effects as those of the first embodiment can be obtained.

  In addition, according to the second embodiment of the low-pressure steam turbine structure of the present invention described above, a smooth flow path can be easily formed, and shape inspection is also easy. The manufacturing cost can be suppressed compared to

Hereinafter, a fourth embodiment of the low pressure steam turbine structure of the present invention will be described with reference to the drawings. FIG. 7 is a conceptual diagram showing the boundary layer velocity distribution with respect to the flow position of the exhaust diffuser of the steam turbine which is the fourth embodiment of the low pressure steam turbine structure of the present invention.
In FIG. 7, the same reference numerals as those shown in FIG. 1 to FIG.

  The fourth embodiment of the low-pressure steam turbine structure of the present invention shown in FIG. 7 is composed of almost the same equipment as the first embodiment, but differs in the following configuration. In the present embodiment, the diameter of the most reduced diameter portion, which is the minimum diameter in the reduced diameter portion G formed on the inner peripheral side wall surface of the flow guide portion 9 constituting the exhaust diffuser 16, is viewed from the turbine axis direction. It differs in that it is formed to have different values depending on the position in the direction.

  In general, there is a pressure recovery coefficient as a parameter related to the determination of the performance of the exhaust diffuser. The pressure recovery factor depends on the exhaust diffuser geometry as well as the inlet conditions. Inflow conditions such as Mach number, pressure profile, and turbulence level depend on the design and operating conditions of the turbine. For this reason, a pressure recovery coefficient and a rotor blade exit static pressure differ with the difference in the position of the circumferential direction seeing from the turbine shaft direction. In this case, since the desired effect may not be obtained if the diameter-reduced part G is configured with the same diameter of the most diameter-reduced part in the circumferential direction, the most contracted in consideration of these values at the circumferential position. The value of the diameter of the diameter part is determined.

In FIG. 7, the steam main flow path outer peripheral surface 10 a of the reduced diameter portion G indicated by a broken line indicates, for example, the position at the lowermost position in the circumferential direction, which is a portion where the pressure recovery coefficient is large and the moving blade outlet static pressure is low. In this case, since the velocity of the steam leak flow 13 becomes larger than that in the other circumferential directions, the value of the diameter of the most contracted diameter portion is set to Rb _max that is larger than the value in the other circumferential directions. This suppresses excessive interference when the steam leak flow 13 and the steam main flow 14 are joined by the reduced diameter portion G, and reduces the separation potential at the outlet of the exhaust diffuser 16.

On the other hand, the steam main flow path outer peripheral surface 10b of the reduced diameter portion G indicated by the solid line shows, for example, the one at the position in the uppermost circumferential direction, which is a portion where the pressure recovery coefficient is small and the moving blade outlet static pressure is high. In this case, since the velocity of the steam leak flow 13 is smaller than that in the other circumferential directions, the value of the diameter of the most contracted diameter portion is set to Rb _min that is smaller than the value in the other circumferential directions.

  According to the fourth embodiment of the low-pressure steam turbine structure of the present invention described above, the same effects as those of the first embodiment can be obtained.

  Moreover, according to the fourth embodiment of the low-pressure steam turbine structure of the present invention described above, the diameter of the most reduced diameter portion in the reduced diameter portion G is formed to have different values at the circumferential position. A larger diffuser effect can be obtained.

  The present invention is not limited to the above-described first to fourth embodiments, and includes various modifications. The above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to the one having all the configurations described.

DESCRIPTION OF SYMBOLS 1 ... Static blade, 2 ... Moving blade, 3 ... Outer peripheral side diaphragm, 4 ... Inner peripheral side diaphragm, 5 ... Turbine rotor, 6 ... Cover, 7 ... Inflow steam flow direction, 8 ... Outflow steam flow direction, 9 ... Flow guide DESCRIPTION OF SYMBOLS 10 ... Steam main flow-path outer peripheral surface, 11 ... Turbine center axis | shaft, 12 ... Bearing cone, 13 ... Steam leak flow, 14 ... Steam main flow, 15a-d ... Boundary layer velocity distribution, 16 ... Exhaust diffuser, 17 ... Dead water area.

Claims (4)

A turbine rotor,
A plurality of last stage blades provided in the circumferential direction of the turbine rotor;
A cover provided at the outer peripheral tip of the final stage rotor blade;
A low-pressure steam turbine structure provided with an exhaust diffuser that is provided downstream of the last stage rotor blade and forms a steam main flow path;
The meridional shape of the inner peripheral side wall surface of the exhaust diffuser corresponding to the outer peripheral surface of the main steam channel is formed in an S-shape that protrudes toward the inner peripheral side in the downstream direction from the downstream end of the cover. And
And the outer peripheral side from the line segment obtained by extending the radial direction position of the innermost peripheral side of the outer peripheral surface of the steam main flow channel downstream from the downstream end of the cover in the tangential direction of the inner peripheral surface of the cover A low-pressure steam turbine structure characterized by being provided in The low pressure steam turbine structure according to claim 1,
The meridian shape of the inner peripheral side wall surface of the exhaust diffuser formed in an S shape that protrudes toward the inner peripheral side toward the downstream direction from the downstream end of the cover is formed by a plurality of arcs. Low pressure steam turbine structure. The low pressure steam turbine structure according to claim 1,
The value of the diameter of the most reduced diameter portion, which is the smallest diameter among the reduced diameter portions of the meridional shape of the inner peripheral side wall surface of the exhaust diffuser formed in an S shape that is convex on the inner peripheral side, is determined from the turbine axial direction. The low-pressure steam turbine structure is characterized by being formed differently depending on the position in the circumferential direction when viewed. The low pressure steam turbine structure according to claim 3,
In the circumferential position viewed from the turbine axis direction and at a position where the value of the pressure recovery coefficient is large, the diameter value of the most contracted diameter portion is larger than the diameter values at other circumferential positions. A low-pressure steam turbine structure.

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